This invention relates to limiting electrical degradation of all-dielectric self-supporting (ADSS) cables.
When ADSS cables are suspended from towers from which power transmission lines are also suspended, they can suffer from electrical degradation caused by dry band arcing.
Dry band arcing arises since there is an induced voltage gradient along the length of the ADSS cable and although this voltage gradient does not present a problem if the cable is dry, if the cable surface becomes electrically conductive due to moisture thereon a current is drawn along the length of the cable. This current is greatest at the tower where the cable is suspended and at earth potential and will dry the cable surface leading to a break in the previously continuously electrically conductive surface. This break can extend around the cable to form a dry band and arcing across the dry band can degrade the cable.
An object of this invention is to limit such dry band arcing adjacent the earthed suspension regions of ADSS cables.
Briefly this is achieved by maintaining the outer surface of the cable dry over a portion thereof adjacent the earthed suspension region.
In an article entitled Arc control devices for use on all-dielectric self-supporting optical cables"" in IEE Proceedings-A, Vol. 140, No. 5, September 1993 to which reference is directed there is disclosed a rain shield for shielding an ADSS cable from rain adjacent an earthed suspension region. This rain shield comprises a cylinder fitted to the cable adjacent the earthed suspension region so that the cable runs along the axis of the cylinder. The end of the cylinder closest to the earthed suspension region is closed and the opposite, or span, end is open. It is stated that if the ratio of the shield length to the annular gap between the shield is high enough, say by a factor of ten, ingress of rain and the deposition of pollutant will be successfully prevented. However, it is acknowledged that atmospheric condensation such as fog or dew will still be deposited on the inside of the shield. Such condensation will also be deposited on the cable inside the shield providing conditions for dry band arcing.
The present invention enables the provision of a more certain way of preventing the cable adjacent the earthed suspension region becoming wet either through rain or condensation.
The invention provides a device for use adjacent an earthed suspension region of an all-dielectric self-supporting cable suspended in an electric field at said region from a tower or the like for limiting dry band arcing adjacent said region, said device comprising means for defining a sleeve for fitting to the cable adjacent said region and means for defining two closures for the sleeve, which closures are engageable with the cable at longitudinally spaced apart locations for forming with the cable and sleeve a closed zone between the closures.
The invention also includes an installation comprising an all-dielectric self-supporting cable suspended at an earthed suspension region thereof in an electric field, from a tower or the like having a device as defined in the last preceding paragraph fitted to the cable adjacent said earthed region.
Preferably, the device is axially split for ease of installation and in this case the means for defining the sleeve comprises at least two hinged or separate members fittable together to form said sleeve. In this case, the two closures may comprise respective elements connected to said members.
The device advantageously further comprises clamp means for clamping said members together when they are fitted together to form said sleeve.
Each of said closures may be provided with or comprise a resilient seal means for sealingly engaging the cable when said sleeve is fitted to said cable. At least one, and preferably each, of said seals means may comprise a plurality of deformable fins for sealingly engaging the cable at longitudinally spaced apart locations.
In each of the illustrated embodiments, one of said closures is provided at one end portion of said sleeve and the other of said closures is provided intermediate said one end portion and the opposite end portion of the sleeve, which opposite end portion is open. In one of these embodiments the open end portion flares radially outwardly towards its free end.
Advantageously, the external surface of at least a portion of said sleeve comprises radially extending surface portions. To this end, said portion of the sleeve may have the configuration of a bellows.
Preferably, at least the means for defining said sleeve are formed from a hydrophobic, electrically insulating material.
The invention also includes an installation comprising an all-dielectric self-supporting cable suspended at an earthed suspension region thereof in an electric field from a tower or the like having a device comprising a sleeve and two closures therefor fitted to the cable adjacent said region, said closures engaging the cable at two spaced apart locations therealong and forming with the cable and sleeve a closed zone between the closures.
In the illustrated embodiments, one of said closures is provided at an end of said sleeve closest to the earthed suspension region and the other of said closures is provided intermediate said end and an end of the sleeve remote from the said region, the end of the sleeve remote from said region being open.
The invention also includes a method of limiting dry band arcing adjacent an earthed suspension region of an all-dielectric self-supporting cable suspended at said region in an electric field from a tower or the like, comprising maintaining the outer surface of the cable dry over a portion thereof adjacent the earthed suspension region by sealing off said portion from the environment.
In order that the invention may be better understood, two embodiments thereof, which are given by way of example only, will now be described with reference to the accompanying drawings, in which: